Answer:

This error is often a sign of Google Throttling migrations. When this happens, we recommend activating the Log subjects of failed items in the Advanced Options of your project. Run the migration again to identify which items are actually failing.

If the items failing are calendar events, it could be because multiple calendar items have exactly the same ICalID at the Source. This is usually a sign of either corrupted items, or a third-party application creating calendar events incorrectly -- and in violation of the ICalendar RFC.

If you are encountering this error because of multiple calendar events, there are two paths for resolution:

Remove the duplicated items from the Source that caused the error

OR

Add the Advanced Option SkipOnDuplicateICalUid=1 in your project. You will then need to:

Remove all items from the Destination.

Reset the statistics of your project.

Resubmit for migration.

With the SkipOnDuplicateICalUid=1 option enabled, MigrationWiz will scan all the Source items, and prevent the migration of the duplicated item, which would fail to be created and thus prevent the error.